In a gas turbine engine hot gas components such as the stator heat shield can become damaged or worn during service. Due to the cost of the components, repair can be preferred over replacement.
Repair methods include methods that involve completely removing a through section of a damaged portion of the component and replacing it with a replacement part. For example U.S. Pat. No. 6,508,000 describes a repair method where a 3D featured replacement part having curved surfaces with corners and edges is fitted by transient liquid phase bonding. The shape of the replacement part is used to align and hold the part during bonding. The method therefore may not be suitable where the repair surface is essentially two-dimensional.
U.S. Pat. No. 6,199,746 provides another repair method where a through section of a component defect is removed by creating an aperture with sloping sides. The slope essentially creates a 3D feature enabling the fitting of a similarly side-sloped plug by forcing the plug into the aperture and subsequently bonding it to the component. While this method enables repair of some essentially two-dimensional defective portions, the forcing of the plug into the aperture potentially creates stress regions and/or defects, affecting component integrity.
Removing a through section of a defective portion, as provided in U.S. Pat. No. 6,508,000 and U.S. Pat. No. 6,199,746, means that the replacement part should be able to withstand the full operating stress the component is subject to, and therefore material and bond integrity can be critical. Where the defect does not penetrate through the depth of the component, such that there is no need to remove a through section of the component, it can be desirable to have an alternative repair method.
Laser metal forming can be used to repair damaged portions having two-dimensional shape by building up a metal layer to replace or remove the damaged regions. The method may not suitable where significant metal depth is involved as the process can be expensive and time consuming.
As an alternative, U.S. Pat. No. 6,998,568 describes a two step method that includes brazing a replacement coupon into a cavity made within the damaged component and then completing the repair using laser metal forming to build up coupon depth so as to reestablish wall thickness requirements. Although disadvantaged by being a two-step process, the demands of a high tolerance replacement part can be reduced, and the disadvantage of the laser metal forming process can be reduced although significant time-consuming laser metal forming can still be involved. In addition the coupon should be fixed mechanically, such as by clamping during the brazing step, a difficult task for some complex shaped components, or by tack welded shims that are subsequently removed. This can result in several different repair cycles if more than one defect needs to be repaired, or the method may not be practical for the component involved.